An aluminum plate to be used as a support for lithographic printing plate is etched on the surface thereof with caustic soda before or after being mechanically or electrochemically roughened.
Further, in an electrochemical roughening process nitric acid and hydrochloric acid are used. In an anodic oxidation process, sulfuric acid is used. Thus, various processing solutions are used in the surface treatment of a support.
The variation of the aluminum ion concentration in the processing solution to be used in these surface treatment processes results in a big difference in the configuration of the surface of the support thus treated, exerting a great adverse effect on the quality of the support.
As mentioned above, the aluminum ion concentration in the processing solution has a great adverse effect on the quality of an aluminum support for printing plate. Accordingly, it is important to keep the aluminum ion concentration in the processing solution at a predetermined value.
A conventional common practice of keeping the aluminum concentration in the processing solution constant is to properly discharge the processing solution from the processing system. Another conventional common practice is to cause aluminum ions in the processing solution to be adsorbed by an ion exchange resin as mentioned in JP-A-57-192300 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, the external adjustment method which comprises properly discharging the processing solution from the system to keep the aluminum ion concentration constant is disadvantageous in that it requires the processing solution to be discharged from the system, resulting in a drastic increase in the consumption of the processing solution and the burden of disposal of waste liquor.
An aluminum sash is etched with caustic soda in the process for the matting of the surface thereof or the formation of an anodized film thereon for higher durability. Since this process requires a large amount of an etching solution to be used, it is necessary that aluminum hydroxide be recovered from the etching solution.
Further, various surface treatment processes such as anodic oxidation with sulfuric acid have the same disadvantages as the foregoing process for the surface treatment of an aluminum support for printing plate with respect to the disposal of waste liquor. For example, these surface treatment processes require the recovery of aluminum hydroxide from a sludge produced by the neutralization of waste liquor and the recovery of aluminum ions for the adjustment of the aluminum ion concentration of the surface treatment.
As mentioned above, in the production of alumina, lithographic printing plate or aluminum sash, the procedure of purifying a crude sodium aluminate solution in the course of the process to obtain a high purity aluminum hydroxide whereby the aluminum ion concentration of the surface treatment is properly controlled is important to prepare a high purity aluminum hydroxide from a sodium aluminate solution and recover aluminum hydroxide from an aluminum sludge produced by the process.
In the process for the preparation of alumina from bauxite, the step of obtaining a purified solution of sodium aluminate from a supersaturated sodium aluminate solution to prepare aluminum hydroxide has been heretofore carried out by a sedimentation process which comprises allowing crystalline impurities to be precipitated in a thickener to remove impurities from the supersaturated sodium aluminate solution. However, this process is disadvantageous in that the crystalline impurities are so fine that a prolonged sedimentation time and a large sedimentation area are needed, producing the necessity of a large scale facility. Further, extremely fine crystalline impurities (regarded as almost liquid) cannot be fully removed. Accordingly, it takes time as long as 42 to 72 hours to crystallize aluminum hydroxide. Moreover, the resulting purified sodium aluminate solution has a poor purity. The percent crystallization of aluminate hydroxide is not too good.
In order to separate sodium and aluminum ions from a sodium aluminate solution, diffuse dialysis using an ion exchange membrane previously proposed by the inventors may be employed to discharge aluminum ions out of the system (JP-A-1-200992). However, the diffuse dialysis is disadvantageous in that insoluble metals such as iron attached to the membrane must be removed and a scale such as aluminum hydroxide attached to the membrane on the waste liquor side thereof after diffuse dialysis must be removed once per three days by washing with an acid such as sulfuric acid, causing a rise in the running cost.
As a crystallization process for the preparation of crystalline aluminum hydroxide and a sodium aluminate solution from a supersaturated sodium aluminate solution, particularly a crystallization-process which comprises crystallizing aluminum hydroxide from a supersaturated sodium aluminate solution while the solution used is being reused as an etching solution at an etching step to keep the aluminum ion concentration of the etching solution or the like at a predetermined value, a process as disclosed in JP-A-5-279020 is known.
However, when a particulate crystalline aluminum hydroxide (normally finely divided particles having a size of from 1 .mu.m to 150 .mu.m unless otherwise specified) as a common industrial product or a crystalline aluminum hydroxide obtained according to a method disclosed in JP-A-5-279020 is circulated in the form of slurry by means of a pump, an external force developed by the pump may break some secondary crystals to primary crystals or further reduce the size of the primary crystals. Crystalline aluminum hydroxide may be hereinafter occasionally referred simply to as "aluminum hydroxide" unless confused.
Referring to the use of aluminum hydroxide as an abrasive, an industrially available common aluminum hydroxide or aluminum hydroxide obtained according to the method disclosed in the above cited JP-A-5-279020 shows an extremely short abrasive life as compared with other abrasives and thus must be used in a huge amount.